Tuesday's US Supreme Court decision upholds a ban on affirmative action in college admissions only in Michigan. Experts say it opens the door for lawmakers or voters in other states to establish bans of their own.

Graduating students listen to President Barack Obama speak at the 2010 University of Michigan commencement ceremony. In her dissenting opinion in Tuesday's ruling, Supreme Court Justice Sonia Sotomayor cited a study showing that Michigan's affirmative action ban has reduced minority degree attainment at the University of Michigan. In 2012, 4.4 percent of its students receiving undergraduate degrees were black – the lowest level since 1991.

The US Supreme Court decision Tuesday upholding a Michigan voter initiative that bans the use of race-based admission policies at the state’s public universities may not have immediate impact for colleges outside of Michigan, but it gives a go-ahead signal to other states contemplating similar initiatives –and sends a message about where America's highest court stands on affirmative action.

“This decision provides room for opponents of affirmative action to move forward,” says Richard Kahlenberg, a senior fellow at the Century Foundation in Washington. “This is a step away from using race [in admissions]. It’s not a leap, but it’s a step. Now public universities will increasingly worry about whether a state initiative will come onto the ballot in their state.”

Michigan, whose voters approved Proposal 2 in 2006, has already effectively ended affirmative action at public colleges. It is one of eight states that have instituted such policies. Six states – California, Washington, Michigan, Nebraska, Arizona, and Oklahoma – have banned race-based admission policies in public higher ed via ballot initiative. (Colorado is the only state in which such an initiative was defeated, in 2008, by a razor-thin margin.) Florida used an executive order to ban the use of racial preferences in state employment and higher-ed admissions, and the New Hampshire legislature has also outlawed race-based affirmative action at public colleges.

Two more states, Georgia and Texas, banned the use of race in public college admissions for a time after lower-court decisions, and after the US Supreme Court in 2003 cleared them to consider race as a factor, top schools in those states chose not to reinstate affirmative action programs.

Taken together, says Mr. Kahlenberg, states that ban any form of affirmative action in admissions educate at least one-quarter of the US student population.

But Kahlenberg also says such bans do not need to lead to less diversity at public universities.

In a 2012 study, he examined the states where race had been banned as a factor in college admissions and looked at the race-neutral policies they used instead. Most states used alternatives based primarily on family income of a student. At 7 of 10 universities he studied – University of Texas in Austin, Texas A&M, University of Washington, University of Florida, University of Georgia, University of Nebraska, and University of Arizona – the percentages of African-American and Latino students in the student body met or exceeded the percentages back when race was a factor in admissions.

“When universities are banned from using race, they don’t simply give up on diversity. Instead they try to find ways to creatively promote diversity by reaching out to economically disadvantaged students of all races, which is something I think they should have been doing all along,” says Kahlenberg. “Today, obstacles students face are more closely aligned with class than with race.”

The University of Texas in Austin, for instance, gives students preference in admission on the basis of several factors, including socio-economic status, whether they come from single-parent homes, the language spoken at home, family responsibilities, socio-economic status of the high schools they attended, and the average SAT or ACT score of the school attended in relation to the student’s scores. The university also adopted a “top 10 percent” plan, in which the top 10 percent of every high school class is automatically admitted, giving a leg up to students from more disadvantaged schools, who might have lower test scores.

Under the new policies, the share of African-American students at UT-Austin rose from 4.1 percent in 1996 (when race was a factor in admissions) to 4.5 percent in 2004, and the representation of Hispanics rose from 14.5 percent in 1996 to 16.9 percent in 2004.

The three cases Kahlenberg studied where diversity decreased – University of California at Berkeley, University of California in Los Angeles, and University of Michigan – were all highlighted Tuesday by Supreme Court Justice Sonia Sotomayor in her dissenting opinion.

Justice Sotomayor cited a study showing that Michigan's affirmative action ban has reduced minority degree attainment at the University of Michigan. In 2012, 4.4 percent of its students receiving undergraduate degrees were black – the lowest level since 1991.

She noted that at UCLA the percentage of underrepresented minorities plummeted after California’s 1998 affirmative action ban, from 30.1 percent of the entering class in 1995 to 14.3 percent in 1998. “The elimination of race-sensitive admissions policies in California has been especially harmful to black students,” wrote Sotomayor. “In 2006, for example, there were fewer than 100 black students in UCLA’s incoming class of roughly 5,000, the lowest number since at least 1973.”

"We are fortunate to live in a democratic society," Sotomayor wrote in a passionate 58-page dissent. "But without checks, democratically approved legislation can oppress minority groups. For that reason, our Constitu­tion places limits on what a majority of the people may do. This case implicates one such limit: the guarantee of equal protection of the laws."

Although most states still allow race to be considered as a factor in college admissions, it is likely that affirmative action will face new challenges in court and at the ballot box.

In his majority opinion for the high court's 6-to-2 decision, Justice Anthony Kennedy emphasized that the ruling is not about the merits of affirmative action, but simply about whether voters have the right to determine whether it can be used in decisions such as college admissions. “This case is not about how the debate about racial preferences should be resolved. It is about who may resolve it,” Justice Kennedy wrote.

But Kahlenberg notes that Kennedy – always closely watched because he is often the a swing vote on the court – did signal in the opinion his distaste for race-based affirmative action, using language suggesting that it can cause resentment and is something the nation should try to move past.

“In a society in which those [racial] lines are becoming more blurred, the attempt to define race-based categories also raises serious questions of its own,” Kennedy wrote. “Thus could racial antagonisms and conflict tend to arise in the context of judicial decisions as courts undertook to announce what particular issues of public policy should be classified as advantageous to some group defined by race.”

Already, in last year’s case concerning the University of Texas, the court allowed race-conscious admissions policies but admonished schools to explore alternatives, and prove they don’t work, before using race.

Such language in Kennedy’s decision Tuesday sends yet another signal about the Supreme Court’s thinking on the issue, says Kahlenberg.

“As people look at where the court is on affirmative action, supporters of affirmative action ought to be concerned,” he says.